Paperboard is used in various packaging applications. For example, coated unbleached kraft is used to package beverage containers, frozen foods, cereals and a wide variety of other food and non-food consumer goods. Other forms of bleached and unbleached coated paperboard are used for a variety of packaging options in food service and consumer products.
Accordingly, there is a demand for paperboard that has a generally smooth surface with few imperfections to facilitate the printing of high quality text and graphics, thereby enhancing the shelf presence and brand appeal of products packaged in paperboard.
Conventionally, paperboard smoothness is achieved by mechanically pressing the sheet in a series of rolls or calenders. Calendering can occur before and after coating, and may be achieved by a variety of means. For example, wet stack calendaring is typically used prior to coating and utilizes water boxes to supply water to moisten the surface of the board, making the fibers more easily compressed. Hot roll calendaring may also be performed, using either hard or soft heated rolls to plasticize the fibers in the surface. However, mechanically pressing the sheet results in a decrease in the thickness (caliper) of the sheet structure, thereby resulting in a decrease in desired mechanical properties (e.g., stiffness) or requires the use of additional fiber in the sheet (i.e., increase basis weight) to maintain bulk.
Alternatively, manufacturers have attempted to smooth the surface of paperboard by coating the entire surface with a basecoat comprised of various pigments, such as clay, calcium carbonate, TiO2 and the like, then overcoating this base with a second and sometimes even a third coating referred to as a topcoat. Various combinations of basecoat pigments have been demonstrated in the prior art, such as blends of hydrous kaolin clays. Along with hydrous kaolin (primarily the mineral kaolinite which has been directly mined), particulate kaolins also exist in calcined and chemically aggregated forms. Chemically aggregated kaolins are produced by treating hydrous kaolin with chemical agents to alter its physical structure and properties, resulting in a material that is lower in density and higher in scattering coefficient. Calcined kaolins are obtained by processing hydrous kaolin at high temperatures, and result in a microstructure resembling that of chemical aggregates. However, both of these processes result in higher pigment costs. Pigment blends must be optimized for the properties desired in the finished paperboard coating, as well as for cost, as the use of relatively high quantities of specialized pigments substantially increases the cost of preparing smooth and highly printable paperboard.
Accordingly, there is a need for a basecoat and associated paperboard structure that maintains paperboard bulk and provides the desired smoothness for high quality printing, while reducing manufacturing cost.